1. Field of the Invention
The present invention relates to a sample surface observation method and, more particularly, to a method of observing a surface of a sample having therein wiring containing an insulating material and an electrically conductive material.
2. Description of the Related Art
Conventionally, a sample surface observation method is known in which an electron beam is irradiated onto a surface of a sample such as a semiconductor wafer or the like so as to acquire a sample surface image by detecting electrons emitted from the sample surface, and detects a defect such as an open defect or a missing defect based on the acquired sample surface image (for example, refer to Patent Document 1).
FIGS. 1A, 1B and 1C are cross-sectional views of a part of a semiconductor wafer in which contact plugs are formed. FIG. 1A illustrates contact plugs 30, which are normal and grounded. In FIG. 1A, three contact plugs 30 are formed in an insulation layer 40. Each of the contact plugs 30 penetrates the insulation layer 40 so as to electrically connects layers above and under the insulation layer 40 to each other. Generally, the contact plugs 30 are formed by filling an electrically conductive material such as tungsten or copper into contact holes formed in the insulation layer 40.
FIG. 1B illustrates contact plugs including the contact plugs 30 and a contact plug 31 of a missing defect. In FIG. 1B, the contact plug 31 in the middle among the contact plugs 30 to be formed originally is not formed and in a state where the contact plug 32 is missed. As such, the missing defect refers to a defect that a contact hole is not formed at a position where it must be formed during, for example, an exposure process or etching process. The missing defect may cause a problem in that layers above and under the contact plug 31 are not electrically connected to each other.
FIG. 1C illustrates contact plugs including the contact plugs 30 and a contact plug 32 of an open defect. In FIG. 1C, the contact plug 32 in the middle among the contact plugs 30 is formed in an incomplete state where an electrically conductive material such as tungsten or copper is filled insufficiently so that the plug does not penetrate the insulation layer 40, and layers above and under the insulation layer 40 are not electrically connected to each other. As such, the open defect refers to a defect that an electrically conductive material such as tungsten or copper is not appropriately filled in a contact hole. The open defect may cause a problem in that the layers above and under the contact plug 32 are not electrically connected to each other or a resistance between the layers above and under the contact plug 32 increases greatly.
According to a conventional sample surface observation method, the missing defect and the open defect illustrated in FIGS. 1B and 1C are detected by acquiring a wafer surface image acquired from a surface of a semiconductor wafer and comparing a position where a contact plug is to be formed with a position corresponding to the normal contact plug 30 to obtain a difference in image brightness (intensity) in the wafer surface image.
FIGS. 2A, 2B and 2C are illustrations of wafer surface images acquired by a conventional sample surface observation method. FIG. 2A illustrates a wafer surface image in which the normally grounded contact plugs 30 are formed as illustrated in FIG. 1A. In FIG. 2A, the normally grounded contact plugs are displayed in the same brightness.
FIG. 2B illustrates a wafer surface image in which the contact plugs 30 and the contact plug 31 of the missing defect illustrated in FIG. 1B are formed. In FIG. 2A, the contact plug 31 of the missing defect at the center is displayed in the same brightness as the surrounding insulation layer 40.
FIG. 2C illustrates a wafer surface image in which the contact plugs 30 and the contact pug 32 of the open defect illustrated in FIG. 1C are formed. In FIG. 2C, a part of the contact plug 32 of the open defect at the center is displayed in a higher brightness than the part of the normal contact plugs 30 because there is a difference in brightness between the defective contact plug 32 and the normal contact plugs 30.
Thus, in the conventional sample surface observation method, the detection and classification of the missing defect and the open defect is carried out based on a difference in a gradation level such as a monochrome image brightness difference in the wafer surface image.
It should be noted that, as a method of detecting a pattern, which is not electrically connected to other parts, formed in a semiconductor device wafer, there is known a testing method of a semiconductor device which can detects an electric abnormality quickly by detecting a change in an amount of secondary electrons due to a potential difference in a pad (for example, refer to Patent Document 2). According to this method, a conductive pattern extending in a row direction and a column direction and a wiring patter having a predetermined conductive part connected to the pattern are provided in a TEG (Test Element Group) area of a semiconductor device. A change in an amount of secondary electrons is detected by scanning the conductive part by an electron beam.
Patent Document 1: Japanese Laid-Open Patent Application No. 2005-235777
Patent Document 2: Japanese Laid-Open Patent Application No. 2007-80987
However, according to the structure recited in the above-mentioned Patent Document 1, there is a problem in that the detection of a missing defect and an open defect may be difficult because a gradient difference of each position is too small (whose difference depending on a material of the contact plug 30 or a kind of the insulation layer 40). Additionally, especially for the open defect, there is a problem in that it is extremely difficult to detect a defect and classify a kind of the defect because there are a case where the portion of the contact plug 32 having an open defect has a brightness higher than the portion of the normal contact plug 30 (changes into darker black) and a case where the part of the contact plug 32 has a brightness lower than the portion of the normal contact plug 32 (changes into brighter white).
That is, in the conventional sample surface observation method, there is a problem in that it is difficult to discriminate any defects from the normal contact plug 30, which makes the defect detection difficult because the detections of the missing defect and the open defect are performed simultaneously in the same condition.
According to the structure recited in the above-mentioned Patent Document 2, it is possible to perform a general test in a manufacturing condition of a semiconductor device by using TEG. However, a specific and individual test of a pattern actually formed in a semiconductor device must be carried out according to a different method, and there is a problem in that the method of the Patent Document 2 is not applicable to testing an entire surface of a semiconductor device.